A water-absorbent resin or a water-absorbent resin composition has been conventionally widely used as a primary component of paper diapers, sanitary napkins, incontinence pads, and other hygienic materials (absorbent articles) for the purpose of absorbing a body fluid (urine, blood, etc.).
The water-absorbent resin composition contains a water-absorbent resin as a primary component. Known examples of the water-absorbent resin composition include: crosslinked products of partially neutralized products of polyacrylic acid, hydrolysates of starch-acrylonitrile graft polymers, neutralized products of starch-acrylic acid graft polymers, saponification products of vinyl acetate-acrylic ester copolymers, crosslinked products of carboxy methyl cellulose, hydrolysates of acrylonitrile copolymers or acrylamide copolymers and crosslinked products of the hydrolysates, crosslinked products of cationic monomers, crosslinked isobutylene-maleic acid copolymers, and crosslinked products of 2-acrylamide-2-methylpropanesulfonic acid and acrylic acid.
Recent paper diapers and like hygienic materials have increasingly high performance and a decreasing thickness. Thickness reduction is attempted while increasing absorption and securing leak prevention by increasing the amount or ratio (mass-equivalent ratio in absorbent article) of the water-absorbent resin composition used.
Hygienic materials containing an increased amount of water-absorbent resin composition in this manner are preferable in view of simply storing more liquid. However, when actually used in a diaper, the water-absorbent resin composition absorbs water, swells, and changes to soft gel in the diaper. This phenomena, or “gel blocking,”reduces absorption and causes leakage. This is undesirable.
Accordingly, the liquid permeability of the water-absorbent resin composition has recently attracted a great deal of attention. Some documents, including patents documents 1 to 7 listed below, report a method of raising liquid permeability by crosslinking the surface of the water-absorbent resin with cations of aluminum or a like multivalent metal. Multivalent metal cations are widely used also to modify the water-absorbent resin other than the improvement of liquid permeability: for example, improvement of powder fluidity upon moisture absorption and improvement of mixing property of an aqueous liquid with the water-absorbent resin.
It is known that aluminum sulfate generally contains Fe in an amount of 0.16 to 11.5 mass % to the Al (see, non-patent document 1, for example).    Patent document 1: U.S. Pat. No. 6,620,889, Specification (issued Sep. 16, 2003)    Patent document 2: U.S. Pat. No. 6,605,673, Specification (issued Aug. 12, 2003)    Patent document 3: U.S. Pat. No. 6,863,978, Specification (issued Mar. 8, 2005)    Patent document 4: U.S. Pat. No. 6,323,252, Specification (issued Nov. 27, 2001)    Patent document 5: U.S. Pat. Application 2005/00671 (published Mar. 31, 2005)    Patent document 6: U.S. Pat. No. 4,771,105, Specification (issued Sep. 13, 1988)    Patent document 7: U.S. Pat. No. 4,043,952, Specification (issued Aug. 23, 1977)    Non-patent document 1: EUROPEAN STANDARD, EN 878:2004, page 10 (Table 5), published June 2004.